lint(track_config): check exercise directory names are in config (#426)
lint(track_config): check exercise directory names are in config (#426)

With this commit, `configlet lint` now produces an error if there is
a subdirectory in the `exercises/concept` directory or
`exercises/practice` directory of a track that does not have a
corresponding entry in the track-level `config.json` file.

exercism/configlet